1. Field of the Invention
This invention relates to a horizontal synchronizing signal separation circuit for a display apparatus such as a liquid crystal display (LCD) apparatus. More particularly, this invention relates to a horizontal synchronizing signal separation circuit which can extract pulses indicative of horizontal synchronizing timing from a composite synchronizing signal which includes a horizontal synchronizing signal and a vertical synchronizing signal.
2. Description of the Prior Art
In a television system such as the NTSC (National Television System Committee) system or PAL (Phase Alternation Line) system, only a composite synchronizing signal which includes vertical and horizontal synchronizing signals is supplied as a synchronizing signal to a television apparatus.
In a matrix type LCD apparatus which has been widely used in recent years, image information must be sampled once, so a clock signal for the sampling is generated. When a display is performed based on the image information using the above-mentioned television system, the clock signal is required to be accurately synchronized with a horizontal synchronizing signal. FIG. 12 shows a PLL (Phase-Locked Loop) circuit 100 for generating the clock signal. The PLL circuit 100 has a loop which comprises a voltage controlled oscillator (VCO) 101, a frequency divider 102, a phase comparator 103 and a low-pass filter (LPF) 104. It is desirable that a synchronizing signal Sync input into the PLL circuit 100 consists of a horizontal synchronizing signal only. Practically, however, the above-mentioned composite synchronizing signal is supplied to the PLL circuit 100 without being separated, as the synchronizing signal Sync.
FIGS. 13A to 13C show composite synchronizing signals used in the NTSC system. FIG. 13A shows a composite synchronizing signal in a transition from an even field to an odd field. FIG. 13B shows part of a composite synchronizing signal in one field. In FIG. 13C, a composite synchronizing signal in a transition from an odd field to an even field is shown. As shown in FIGS. 13A to 13C, in a transition from a certain field to the next field, a composite synchronizing signal includes vertical synchronizing pulses and equalizing pulses 22 as well as horizontal synchronizing pulses 21. The equalizing pulses 22 are inserted so that the waveforms of the composite synchronizing signal in the portion of the vertical synchronizing signal and in the peripheral portion thereof are equalized during the transition from the even field to the odd field and during the transition from the odd field to the even field. In the peripheral portion of the vertical synchronizing signal, the width of each of the horizontal synchronizing pulse 21 and equalizing pulse 22 is half as compared with that of the usual horizontal synchronizing pulse 21.
In the prior art, since such a composite synchronizing signal is input into the PLL circuit 100 (FIG. 12) without being separated, the phase relationship in the PLL circuit 100 is disturbed due to the presence of the vertical synchronizing pulses and equalizing pulses in the composite synchronizing signal shown in FIGS. 13A and 13C. This disturbance causes the oscillation frequency of the VCO 101 to fluctuate. If the fluctuation in the oscillation frequency of the VCO 101 is not absorbed even when entering a display period during which image information for the display area is supplied to the LCD apparatus, a serious problem arise in that the resulting image is distorted.
In order to prevent the image distortion from occurring, the fluctuation in the oscillation frequency of the VCO 101 must be absorbed during a period prior to the display period (i.e., a vertical retrace line interval). This mainly makes it difficult to design a PLL circuit for a matrix type display apparatus such as an LCD apparatus. Especially in a specific reproduction of a video tape such as a rapid reproduction, the waveform of the composite synchronizing signal largely differs from its usual waveform. In such an occasion, therefore, it is very difficult to cope with such a composite synchronizing signal.
In some prerecorded video tapes which are commercially available, an AGC (Auto Gain Control) signal for video recording is intentionally inserted into a composite video signal for the purpose of unstable reproduction of a duplicated video tape of the prerecorded video tape. When the duplicated video tape is reproduced and composite synchronizing signals are to be extracted from the composite video signal, the AGC signal cannot be completely removed by a low-pass filter. Therefore, pulses such as a spurious synchronizing signal mixedly appear immediately after the vertical synchronizing signal in the composite synchronizing signal, as shown in FIG. 14. If the composite synchronizing signal which includes the spurious synchronizing signal immediately before the display period as shown in FIG. 14 is to be input, the PLL circuit 100 is disturbed by this spurious synchronizing signal, resulting in that it cannot be practically stabilized before the display period. In the prior art, although the image area on the screen to be displayed is made smaller for solving the problem, it is difficult to completely cover up the image distortion appearing in the upper portion on the display screen, and in many cases, good display cannot be attained.